This invention relates to new thermoplastic polyurethanes, their preparation and their use as coating compounds.
Polyurethane systems for coating textiles, leather, paper and similar substrates have been known for a long time (see e.g. Saunders and Frisch, Polyurethanes, Chemistry and Technology, Part 1 pages 7-9). Thermoplastic polyurethanes which can be processed as solvent free compounds on calenders or melting and laminating rollers are also known. In comparison to polyurethanes which have to be processed from solution they have the advantage of requiring no solvent and therefore they do not pollute the environment with solvent vapors. Thermoplastic polyurethane elastomers have nevertheless been used to only a limited extent in the past because it is very difficult to obtain products with a wide melting range. It is therefore necessary to control the operating temperature very accurately in order to prevent viscosity variations in the solvent-free melt. Although thermoplastic polyurethane elastomers which contain soft segments of polyesters or polyethers with an average molecular weight below 1000 have sufficient mechanical strength, their low temperature behavior is unsatisfactory and they harden in cold weather. Polyols with molecular weights above 1500 and preferably above 2000 are therefore generally used to improve the low temperature behavior. Such higher molecular weight compounds, however, give rise to other disadvantages in that the disturbing effect of monofunctional impurities increases with the chain length of the polyol so that the articles produced have reduced mechanical strength and occasionally there is an efflorescence of low molecular weight urethane constituents on the surface of the products.
Another disadvantage of the previously known polyurethane elastomers used as thermoplastic materials is their high elasticity. This gives rise to trouble in many fields of application, for example, textiles coated with these substances make a loud rustling noise and crackle when used as garments and, moreover, have a rubber-like handle.
It is an object of this invention, therefore, to produce thermoplastic elastomers which have excellent resistance to low temperature and melt uniformly within a wide temperature range, i.e., they have a melt viscosity which is practically independent of the temperature over a range of more than about 10.degree.C. In addition, it is desired to prevent the efflorescence of low molecular weight constituents on the surface of thermoplastic elastomer coatings and to obtain coatings which have a leather-like handle and low recoil elasticity.